Building-product mixtures, especially concrete and mortar mixtures, are frequently modified with rheology-control additives to keep the mixture flowable and hence efficiently workable. These additives, which are also called superplasticizers, are described in the art and are usually based on polycarboxylate ethers or sulphonates of lignin (resins), melamine (resins) or (poly)naphthalene (resins), and are usually added in aqueous solution to the building-product mixture at the mixing stage. A disadvantage with the use of superplasticizers is the higher entrapment of air into the building-product mixture and hence also in the hardening concrete and mortar. This can lead to deficiencies of mechanical strength and weatherability. Therefore, defoamers or deaerators are often admixed as further additives in order that air entrapment may be reduced.
Defoamers used in the art have the disadvantage that they are frequently water-insoluble and hence do not become distributed uniformly in the aqueous systems. A mixed-additive formulation comprising a superplasticizer and a defoamer is frequently a cloudy mixture which, in relatively short order, can phase-separate between the polar superplasticizer and the apolar, interface-occupatory defoamer. Even worse, an emulsion may not even be formulatable in the first place.
The art does not disclose any siloxane-based water-soluble defoamer for hydraulically hardenable building products which is miscible with superplasticizers in the relevant concentration range and which does not divide into separable phases.
The problem addressed by the present invention was therefore that of providing water-soluble compounds which are useful as defoamers in hydraulically hardenable building products and have good compatibility with superplasticizers.
It was found that, surprisingly, this problem is solved by water-soluble siloxanes having at least two different polyalkoxylene moieties.
This is an unforeseeable surprise to a person skilled in the art particularly because hydrophilic organomodified siloxanes tend to be used more for foam stabilization than for defoaming, as described for example in patent application DE 10 2008 043343.8 (US 2010 0113633 A1).